1. Field of the Invention
The present invention relates to an encoder that detects a displacement of an object to be measured.
2. Description of the Related Art
Previously, as a displacement detection apparatus that detects a displacement such as a moving amount or a rotating amount of an object to be measured, an encoder has been known. As the encoder, there is an optical encoder, a magnetic encoder, or the like. For example, the optical encoder is configured by including a light source, a scale that can be displaced relative to the light source, and a light receiving element that receives light that is reflected on or transmits through the scale. A pattern that reflects or transmits through the light is formed on the scale, and in accordance with the relative displacement of the scale, a light intensity of the light received by the light receiving element is changed. The encoder detects the displacement of the object to be measured based on a detection signal outputted from the light receiving element in accordance with the change of the light intensity.
U.S. Pat. No. 6,637,118 discloses an encoder that includes a cylindrical body and a scale attached to the cylindrical body. In the configuration of U.S. Pat. No. 6,637,118, the scale is held on an inner surface of the cylindrical body, and both ends of the scale are fixed using screw members. A center axis of the screw member that fixes the scale is decentered. Therefore, even when a length of the scale is slightly different from a design value, the scale can be attached to the cylindrical body.
However, in the configuration disclosed in U.S. Pat. No. 6,637,118, when the scale is expanded or contracted by an environmental change such as a temperature change or a humidity change, the scale is deflected or a stress is applied to the scale, and therefore a shape of a reflection surface in a signal detection effective region of the scale may be deformed. When the deflection of the scale or the deformation of the scale surface is generated, a detection position (a detection radius position of the cylindrical body) by the sensor unit is changed, and therefore a detection rotational angle of the cylindrical body contains an error. In the optical encoder, by the change of the angle of the scale surface caused by the deflection of the scale or the deformation of the shape of the scale surface, an optical path of the light source, the scale surface, and the sensor is shifted from an ideal optical path. This shift of the optical path causes an error of the detection position in a circumferential direction of the cylindrical body.
Thus, when the deflection of the scale or the deformation of the shape of the scale surface (the expansion or the contraction of the scale) is generated by the change of the environmental condition, a detection accuracy of the encoder is deteriorated by the detection error of the rotational angle of the cylindrical body.